Whenever you have a problem, please post a little sample data (CREATE TABLE and INSERT statements, relevant columns only), so that the people who want to help you can re-create the problem and test their ideas.

Also post the results you want from that data, and an explanation of how you get those results from that data, with specific examples.

If you're asking about a DML statement, such as UPDATE, the CREATE TABLE and INSERT statements should re-create the tables as they are before the DML, and the results will be the contents of the changed table when everything is finished.

Always say which version of Oracle you're using (for example, 11.2.0.2.0).

LAG

The LAG function is used to access data from a previous row. The following query returns the salary from the previous row to calculate the difference between the salary of the current row and that of the previous row. Notice that the ORDER BY of the LAG function is used to order the data by salary.

That example orders the data by salary - you would order yours by date.